There are many applications in clinical and research medicine in which measurement or estimation of volumetric blood flow within a blood vessel is desirable. One method of making such measurements uses ultrasonic Doppler techniques to measure blood flow velocity and thereby estimate volumetric blood flow. Velocity of an object is often measured using the Doppler effect. Single frequency ultrasonic energy is transmitted into an area of tissue containing the blood flow to be measured. This insonification of the area is typically referred to as illumination. Resulting ultrasonic energy is reflected, or backscattered, from the illuminated area. Energy reflected from moving targets, such as fluid and blood cells, will be shifted in frequency from the illuminating frequency according to the well-known Doppler effect. The Doppler shifted frequency provides a measure of the blood flow velocity.
In clinical and research applications, it is often necessary to study blood flow for an extended period of time. Thus, in ambulatory living organisms, such as animal or human subjects, there is a need in the art to provide a battery-powered ultrasonic Doppler blood flow meter for measuring blood flow velocity for an extended period of time, allowing a human or animal patient freedom of movement during the study and minimizing the need for supervision by the clinician. There is also a need in the art to provide a small, low-power ultrasonic Doppler blood flow meter that is suitable for implantation in a human or animal subject. There is a further need in the art to provide an implantable ultrasonic Doppler blood flow meter that maintains adequate signal-to-noise (SNR) ratio for accurate velocity estimation.